Knee Point Voltage for CT

Thayalan, it depends on the class of the CT (ie - protection or metering)

For a METERING CT (eg 200/5 CT class 0.5M 15VA), knee-point can be calculated by:

Vapp = VA / Ian x F (F is Rated Accuracy Limit Factor)

Imax = 5 x 0.5/100 = 25mA

Vapp = 15 / 5 = 3V

For a PROTECTION CT (Class P & PR)

eg 400/5 CT Class 2.5P 200 F20 with 1 ohm secondary resistance

Imax = 20 x 5 x 2.5 /100 = 2.5A

Vapp = 200 + 20 x 5 x 1 = 300V

Or a PROTECTION CT (Class PL or PX)

eg 1000/1 CT Class 0.05 PL 950 R3

Imax = 0.05A at 950V, Wind resistance max = 3 ohms

Hope this helps

Justin

Vide IS 2705, Part4 - 1992:

3.1.2

Minimum Knee-Point Voltage (Vk), specified in accordance with a formula of the type:

VK = K. IS (RCT + RB)

where

VK is the minimum knee-point voltage in volts,

K is a parameterto be specified by the purchaser which depends on the system fault level and the characteristics of the relay intended to be used,

IS is the rated secondary current of the current transformer (or the secondary current as derived from a specified turns ratio and primary current),

RCT is the resistance of the secondary winding corrected to 75 deg.C

RB is the impedance of the secondary circuit as specified by the purchaser.

In the above formula, the purchaser shall specify the values of 'K', 'IS and 'RB'. The value of 'RCT shall be to the discretion of the manufacturer.

NOTE-It is recognized that for certain special requirements, a limitation on the value of 'RCT,' may be required. In such cases, the purchaser may specify a maximum value for 'RCT' at 75 deg.C. In other cases, the choice of 'RCT' should be left to the manufacturer.

This can be tested at site by performing the Magnetistaion Charecteristics TEst on the CT.